According to Consequence of Sound sources, Tame Impala was booked before Ariana to fill Justin Timberlake's slot.
The Kanye fallout was supposedly very last minute and therefore they scrambled for Ariana, they even moved around the dates to fit her in because I think she has shows on the Friday+Saturday.

I still think Tame Impala will be at this. Feel like they're going to make a UK appearance on the festival dates they've been scheduled and this seems like the perfect place for them. Yes they headlined Citadel last year but I don't think it matters that much